docs: def asDictionary(self):
          """ returns the data source as a dictionary """
          return {
              "type": "joinTable",
              "leftTableSource": self._leftTableSource,
              "rightTableSource": self._rightTableSource,
              "leftTableKey": self._leftTableKey,
              "rightTableKey": self._rightTableKey,
              "joinType": self._joinType
          }